1. Field of the Invention
The present invention relates to a method for efficiently combustion-treating combustible waste, and relates to an apparatus used for realizing such a method for combustion treatment.
2. Description of the Related Art
Much of urban refuse and industrial waste are subjected to incineration treatments with incineration equipment and, thereafter, are disposed of in landfills, or are disposed of in landfills with no incineration treatment. However, in recent years, available landfills have been reduced and, therefore, the treatment has approached a limit. Consequently, a technique for performing a gasification treatment and volume reduction of the combustible component by efficiently combustion-treating the combustible waste, for example, waste plastic, has been required.
Herein, in many cases, the combustible waste includes waste containing chlorine, such as a vinyl chloride-based resin, waste containing sulfur, such as vulcanized rubber, and furthermore, waste containing highly volatile metal elements, for example, alkali metals, lead, and zinc, and the like. However, when such waste containing chlorine and/or sulfur is subjected to the incineration treatment, a problem occurs in that a corrosive gas, for example, a chlorine-based gas, hydrogen sulfide, and sulfurous acid gas, is generated during the treatment and, therefore, treatment equipment is corroded and damaged. In particular, it is said that when a compound containing a halogen element, for example, chlorine, is subjected to the incineration treatment, dioxin is generated and, therefore, an influence exerted upon the environment becomes an important problem. Furthermore, when waste containing highly volatile metal elements, for example, alkali metals, lead, and zinc, is subjected to the incineration treatment, a problem occurs in that the metals are volatilized during the treatment and, therefore, these metals adhere to the treatment equipment or form low melting point compounds so as to corrode the treatment equipment. Consequently, periodic maintenance is required.
On the other hand, from the viewpoint of making effective use of the combustible waste as a resource, a technique using the waste plastic and shredder residue among the urban refuse and industrial waste as a fuel of a furnace has been suggested. For example, there is a technique for using the waste plastic and shredder residue as a fuel of a smelting furnace, such as an arc furnace, etc., for manufacturing steel. However, when large quantities of waste plastic and shredder residue are put in at a time, since gasification and combustion are performed in a short time, there have been problems in that the efficiency of heat conduction to iron scrap and the efficiency of reaction with molten steel have been degraded. Furthermore, when gasification and combustion are performed in a short time, control of the combustion temperature and the quantity of unburned gases become difficult and, therefore, the temperature in the furnace may be increased rapidly, and large quantities of unburned gases may be brought about in the furnace. In this case, although combustion is performed in a secondary combustion chamber installed in the latter part of the arc furnace, there have been problems in that when the quantity of the unburned gases has been large, explosion has occurred, or complete combustion has not been achieved and toxic gases have been generated.
In addition, a technique, in which the waste plastic and shredder residue are blown into a blast furnace as a fuel alternative to coke, has been suggested. However, since the waste plastic and shredder residue are mixture of waste having various chemical compositions, heating values are not uniform and, therefore, it has been required to properly control the quantity of blowing in order to keep the heating value stable. However, it has been very difficult to control the quantity of blowing in order to keep the heating value by combustion constant, and a problem has been occurred in that the heating value has been increased excessively and, therefore, the inside of the furnace has been damaged.
Regarding effective use of the combustible waste as the resource, when combustible waste containing chlorine, such as a vinyl chloride-based resin, combustible waste containing sulfur, and combustible waste containing a highly volatile metal element, etc., are put in the furnace, since the inside of the furnace is damaged by an exhaust gas generated, and adhesion of a metal, or the like is brought about, it has been necessary to remove beforehand the combustible waste containing such an element. However, such a separation is very difficult in practical operation.
In recent years, a method, in which the combustible waste is treated using a gasification melting furnace, has been suggested. However, for example, regarding a shaft furnace type gasification melting furnace, since a melting zone which reaches 1,700° C. to 1,800° C. is located at the bottom portion of the furnace, an auxiliary fuel, e.g., coke and heavy oil, is required and, therefore, the treatment cost is increased. Furthermore, since it is difficult to keep the heating value by combustion constant, there have been problems in that a molten material has been solidified due to shortage of the quantity of heat, or a refractory material has been damaged because of becoming in an overheated condition. On the other hand, a rotary kiln type gasification melting furnace is an external heating type furnace and, therefore, the heat transfer efficiency is low, and it is difficult to heat uniformly. Consequently, tar and undecomposed materials generated by thermal decomposition may cover the heat transfer surface and, therefore, the heat transfer efficiency may be further degraded, or conversely, the refractory material may be damaged because of increase in temperature. Furthermore, there has been a problem in that since the external heating type has a low heat transfer efficiency, upsizing of the furnace cannot be avoided.
A gasification treatment apparatus for waste, which has been provided with a fluidized bed gasification furnace and a melting furnace, has been suggested as an apparatus for producing a combustible gas from waste. In this technique, waste is roughly separated into high-calorie waste and low-calorie waste, and the heating value in the furnace is kept constant by mixing them. However, regarding a mixture of waste having different component compositions, separation in accordance with the exothermic calorie is difficult and, therefore, the problem that a molten material is solidified in the furnace due to shortage of the quantity of heat, or a refractory material is damaged because of excessive increase in temperature cannot be overcome.
Since a high-temperature exhaust gas is generated when the combustible waste is burned, if heat is recovered from the exhaust gas, it becomes possible to make effective use of the resource. However, when the combustible waste is burned, corrosive gases derived from chlorine and sulfur may be generated, gases containing metal elements, for example, alkali metals, lead, and zinc, may be generated and, therefore, heat recovery at a high temperature has been difficult. Consequently, hitherto, it has been attempted to cool initially the high-temperature exhaust gas to on the order of 400° C. to 650° C. and, thereafter, to generate electric power by the use of a gas heated by heat exchange with the resulting cooled exhaust gas. However, the enthalpy of vapor is low and, therefore, upsizing of the boiler has not been able to avoid. When heat is recovered from a corrosive exhaust gas, since chloride, etc., are adhered in the heat exchanger, in many cases, there is no other choice but to adopt a plate type structure and, therefore, cost reduction in the heat exchange apparatus cannot be expected. Furthermore, when the heat exchanger has been used for a long time, it has been necessary to use an expensive material having superior corrosion resistance, for example, stainless steel containing a high portion of Ni and a cobalt alloy, and to clean up at frequent intervals.